CHAPTER 9
Patterns of Inheritance
1. Humans have long relied on selective breeding to produce specialized breeds of
livestock and pets. Mendel’s principles of inheritance are fundamental to the workings
of artificial selection.
2. Variations of Mendel’s laws explain the inheritance patterns of many human traits and
diseases.
3. Determining the probability of genetic diseases by following patterns of inheritance
enables us to anticipate disease and better address its consequences. Pedigrees and
genetic testing are an increasingly common part of modern medicine.
Biology and Society: A Matter of Breeding
1. Explain why purebred dogs are important in genetic research.
Heritable Variation and Patterns of Inheritance
2. Define and distinguish between self-fertilization, cross-fertilization, true-breeding
organisms, hybrids, the P generation, the F1 generation, and the F2 generation.
3. Define and distinguish between the following pairs of terms: heterozygous versus
homozygous, dominant allele versus recessive allele, genotype versus phenotype, and
phenotypic ratio versus genotypic ratio.
4. Define the law of segregation and explain how it applies to reproduction.
5. Define Mendel’s law of independent assortment and explain how it applies to a dihybrid
cross.
6. Explain how a testcross can be performed to determine an organism’s genotype.
7. Explain how and when the rule of multiplication should be used to determine the
probability of an event.
8. Explain how a pedigree is used to determine how a particular human trait is inherited.
Define a carrier and explain how carriers are revealed in human pedigrees.
9. Compare the frequency and method of inheritance of recessive and dominant disorders.
Explain how a dominant lethal allele can be inherited.
Variations on Mendel’s Laws
10. Define and distinguish between complete dominance, incomplete dominance, and
codominance.
11. Describe the selective advantage of people who are heterozygous for sickle-cell disease.
12. Define and distinguish between pleiotropy and polygenic inheritance. Describe
examples of each.
13. Explain how the environment influences the expression of traits.
The Chromosomal Basis of Inheritance
14. Define the chromosome theory of inheritance and explain how linked genes are
inherited differently from nonlinked genes.
15. Explain why researchers used fruit flies and how they created linkage maps.
Sex Chromosomes and Sex-Linked Genes
16. Explain how chromosomes determine the sex of a human.
17. Explain why sex-linked diseases are more common in male humans.
18. Describe the general characters of the following sex-linked disorders in humans: redgreen colorblindness and hemophilia.
Evolution Connection: Barking Up the Evolutionary Tree
19. Describe the relationships between the many breeds of dogs.
Key Terms
ABO blood groups
achondroplasia
alleles
carrier
character
chromosome theory of inheritance
codominance
cross
dihybrid cross
dominant allele
F1 generation
F2 generation
genetics
genotype
hemophilia
heredity
heterozygous
homozygous
Huntington’s disease
hybrid
hypercholesterolemia
inbreeding
incomplete dominance
law of independent assortment
law of segregation
linkage map
linked genes
loci
monohybrid cross
pedigree
P generation
phenotype
pleiotropy
polygenic inheritance
Punnett square
recessive allele
recombination frequency
red-green colorblindness
rule of multiplication
sex-linked gene
sickle-cell disease
testcross
trait
true-breeding
wild-type traits
Word Roots
co = together (codominance: phenotype in which both dominant alleles in a heterozygous
individual are expressed)
di = two (dihybrid: a type of cross that mates varieties differing in two characters)
geno = offspring (genotype: an organism’s genetic makeup)
hemo = blood; philia = love (hemophilia: a human genetic disease caused by excessive
bleeding following an injury)
hetero = different (heterozygous: when an organism has different alleles for a gene)
homo = alike (homozygous: when an organism has the same alleles for a gene)
hyper = excessive (hypercholesterolemia: a condition of incomplete dominance resulting in
elevated blood cholesterol levels)
mono = one (monohybrid: a type of cross between organisms that differ in only one trait)
pheno = appear (phenotype: an organism’s physical traits)
pleio = more; trop = change (pleiotropy: when a single gene impacts more than one
character)
poly = many; gen = produce (polygenic: type of inheritance in which two or more genes
affect a single trait)
Student Media
Activities
Monohybrid Cross
Dihybrid Cross
Gregor’s Garden
Incomplete Dominance
Linked Genes and Crossing Over
Sex-Linked Genes
Biology Labs On-Line
PedigreeLab
FlyLab
BLAST Animations
Single-Trait Crosses
Genetic Variation: Independent Assortment
Two-Trait Crosses
LabBench
Genetics of Organisms
MP3 Tutor
Chromosomal Basis of Inheritance
Process of Science
What Can Fruit Flies Reveal about Inheritance?
Videos
Discovery Channel Video: Colored Cotton
Discovery Channel Video: Novelty Gene
Ultrasound of Human Fetus 1
Relevant Current Issues in Biology Articles
Current Issues in Biology, volume 2 (ISBN 0-8053-7108-7)
Does Race Exist?
Current Issues in Biology, volume 4 (ISBN 0-8053-3566-8)
Founder Mutations
Current Issues in Biology, volume 5 (ISBN 0-321-54187-1)
Seeking the Connections: Alcoholism and Our Genes
Relevant Songs to Play in Class
“I Think I’m Going Bald,” Rush
“Behind Blue Eyes,” The Who (original) or Limp Bizkit (2003 cover)
“Pass the Peas,” James Brown or Maceo Parker
Chapter Guide to Teaching Resources
Heritable Variation and Patterns of Inheritance
Student Misconceptions and Concerns
1. Students might think that dominant alleles are naturally (a) more common, (b) more
likely to be inherited, and (c) better for an organism. The text notes that this is not
necessarily true. However, this might need to be emphasized further in lecture.
2. Students using Punnett squares need to be reminded that the calculations are
expected statistical probabilities and not absolutes. Just as we would expect that any six
playing cards dealt might be half black and half red, we frequently find that this is not
true. This might be a good time to show how larger sample sizes increase the likelihood
that sampling reflects expected ratios.
3. The authors note that Mendel’s work was published in 1866, seven years after Darwin
published Origin of Species. Consider challenging your students to consider whether
Mendel’s findings were supportive of Darwin’s ideas. Some scientists have noted that
Darwin often discussed the evolution of traits by matters of degree. Yet, Mendel’s
selection of pea plant traits typically showed complete dominance. Mendel’s pea traits
did not show the possibility for such gradual inheritance.
Teaching Tips
1. Medical technology raises many ethical issues. Consider asking your students this
practical question. How much routine fetal testing do we want our insurance companies to
cover and at what cost for insurance? Ultrasound, for example, is routinely performed on
pregnant women as a normal part of prenatal care. What other tests should be standard?
Who should decide? Who should pay?
2. This early material introduces many definitions that are vital to understanding the
later discussions in this chapter. Therefore, students need to be encouraged to master
these definitions immediately. This may be a good time for a short quiz to encourage
their progress.
3. Many students benefit from a little quick practice with a Punnett square. Have them
try these crosses for practice: (a) PP × pp and (b) Pp × pp.
4. Understanding dihybrid crosses may be the most difficult concept in this chapter.
Consider spending additional time to make these ideas very clear. As the text indicates,
dihybrid crosses are essentially two monohybrid crosses.
5. Many students have trouble with basic statistics. Give your students some practice.
Consider having them work in pairs, each with a pair of dice (for large class sizes, this
can be done in laboratories). Let them calculate the odds of rolling three sixes in a row
and other possibilities.
6. Students also seem to learn much from Figure 9.13 by analyzing the possible
genotypes for the people whose complete genotype is not known. Consider challenging
your students to suggest the possible genotypes for these people.
7. The 2/3 fraction noted in the discussion of carriers of recessive disorder (and in
Figure 9.14) often catches students off guard, as they are expecting odds of 1/4, 1/2, or
3/4. However, we eliminate the dd (deaf) possibility, as it would not be a carrier. So, the
odds are based out of the remaining three genotypes Dd, dD, and DD.
8. Genetic tests are now available to inform a person whether they have the
Huntington’s allele. The test is especially important to the children of a parent with
Huntington’s disease. Consider asking your class: (1) what are the odds of developing
Huntington’s disease if a parent has this disease (50%) and (2) whether or not they
would want this genetic test. The Huntington Disease Society website (www.hdsa.org)
offers many additional details. It is a good starting point for those who want to explore
this disease in more detail.
9. As a simple test of comprehension, ask students to explain why lethal alleles are not
eliminated from a population. Several possibilities exist: the lethal allele might be
recessive, persisting in the population due to the survival of carriers, or the lethal allele
might be dominant, but is not expressed until after the age of reproduction.
Variations on Mendel’s Laws
Student Misconceptions and Concerns
1. As these variations of Mendel’s laws are introduced, students are likely to get confused
and become uncertain about the prior definitions. Consider keeping a clear definition of
these different patterns of inheritance available for the class to refer to as new patterns are
discussed.
2. In larger classes, the chances increase that at least one student has a family member with
one of the genetic disorders discussed. Some students may find this embarrassing
whereas others might have a special interest in learning more about these potentially
personal topics.
Teaching Tips
1. Incomplete dominance is analogous to a compromise, or a gray shade. The key concept
is that both “sides” have input. Complete dominance is more analogous to an authoritarian
style, overruling others and insisting on things being a certain way. Although these
analogies might seem obvious to us, many students new to genetics appreciate them.
2. Another analogy for cholesterol receptors is fishing poles. The more fishing poles
you use, the more fish you will likely catch. Heterozygotes for hypercholesterolemia
have fewer “fishing poles” for cholesterol. Thus, fewer “fish” are caught and more
“fish” remain in the water.
3. Students can think of blood types as analogous to socks on their feet. You can have
socks that match, a sock on one foot but not the other, you can wear two socks that do
not match, or you can even go barefoot (type O blood)! Developed further, think of
Amber and Blue socks. Type A blood can have an Amber sock with either another
Amber sock or a bare foot (or “zero” sock). Blue socks work the same way. One amber
and one blue sock is the AB blood type. No socks, as already noted, represent type O.
4. Consider specifically comparing the principles of codominance (expression of both
alleles) and incomplete dominance (expression of one intermediate trait). Students will
likely benefit from this direct comparison.
5. The American Sickle Cell Anemia Associations’ website (www.ascaa.org) is a good
place to get additional details.
6. Polygenic inheritance makes it possible for children to inherit genes to be taller, or
shorter, than either parent. Similarly, skin tones can be darker or lighter than either
parent. The environment also contributes significantly to the final phenotype for both of
these traits.
The Chromosomal Basis of Inheritance
Student Misconceptions and Concerns
1. This section of the chapter relies on a good understanding of the chromosome-sorting
process of meiosis. If students were not assigned Chapter 8, and meiosis was not otherwise
addressed, it will be difficult for students to understand the chromosomal basis of
inheritance or linked genes.
2. The nature of linked genes builds on our natural expectations that items that are closely
together are less likely to be separated. Yet, students may find such concepts initially
foreign. Whether it is parents holding the hands of children or people and their pets, we
generally know that separation is more likely when things are farther apart. You might
demonstrate this simply by drawing a line down a page of text. The likelihood that the
line separates any pair of words increases as the distance between the words grows
farther apart.
Teaching Tips
1. Building on the shoe analogy developed in Chapter 8, linked genes are like a shoe and
its shoelaces. The two are usually transferred together but can be moved separately under
special circumstances.
2. Crossing over (from Chapter 8) is like randomly editing out a minute of film from two
movies and swapping them. Perhaps the fifth minute of Bambi is swapped for the fifth
minute of Gone With the Wind. Clearly, the closer two frames of film are together, the
more likely they are to move or remain together.
Sex Chromosomes and Sex-Linked Genes
Student Misconceptions and Concerns
1. The prior discussion of “linked genes” addresses a different relationship than the use of
the similar termed sex-linked genes. The nature of the linkage is quite different. Consider
emphasizing this distinction for your students.
2. The likelihood that at least some students are colorblind in larger classes is very high.
Some of these students might find this interesting and want to discuss it further.
However, others might be embarrassed by what might be perceived as a defect.
Teaching Tips
1. In some ways, sex-linked genes reflect the risk of not having a backup copy of a file on
your computer. If you only have one copy, and it is damaged, you have to live with the
damaged file. Having two X chromosomes in females provides a “backup copy” that can
function if one of the sex-linked genes is damaged.
2. For additional information about hemophilia, consider visiting the website of the
National Hemophilia Foundation at www.hemophilia.org.
Answers to End-of-Chapter Questions
The Process of Science
18. Suggested answer: Start out by breeding the cat to get a population to work with. If the
curl allele is recessive, two curl cats can have only curl kittens. If the allele is dominant, curl
cats can have “normal” kittens. If the curl allele is sex-linked, ratios will differ in male and
female offspring of some crosses. If the curl allele is autosomal, the same ratios will be seen
among males and females. Once you have established that the curl allele is dominant and
autosomal, you can determine if a particular curl cat is true-breeding (homozygous) by
doing a testcross with a normal cat. If the curl cat is homozygous, all offspring of the
testcross will be curl; if heterozygous, half the offspring will be curl and half normal.
19. Suggested answer: You could start by taking one strain of fruit flies, crossing them with
the wild type, and analyzing the offspring. Those offspring could be bred to create the
F2 generation, the F2 generation could be bred to create the F3 generation, and so forth.
This same procedure could be followed to cross each of the other nine strains against
the wild type for multiple generations. Further, the ten strains could be crossed with
each other for multiple generations. By examining the results of such crosses, you
should be able to determine (a) which traits are dominant and recessive, (b) which traits
are sex-linked, and (c) recombination frequencies could be used to create linkage maps.
Biology and Society
20. Some issues and questions to consider: Do biologists actually see the structures and
molecules of cells? What about past evolutionary processes, the origin of life, the physical
appearance or behavior of dinosaurs? In other fields of science, what is the evidence for
atoms, subatomic particles, the formation of stars, the composition of Earth’s interior, and
the past positions of the continents? How clear does evidence have to be before it is
acceptable? What prompts a scientist to propose an explanation? What if more than one
explanation can account for the observations? Is it possible to be absolutely sure that an
explanation is correct? That it is incorrect? What is the place of words like correct,
incorrect, fact, and truth in science? Can some of the facts in this textbook be “wrong”?
21. Some issues and questions to consider: Do you agree with gene testing at any level? Is
testing for genetic diseases in embryos any different from testing for Huntington’s
disease in adults, or from testing two potential parents to see if they are carriers of a
genetic disease? If multiple embryos are created during the in vitro process and only
some can be used, should the “best” ones be selected using gene testing? If you agree
with the process, should guidelines be developed to prevent abuse of the technology so
that parents are not selecting features such as the sex of the child, eye color, or skin
tone?
Additional Critical Thinking Questions
The Process of Science
1. Two parents consult a genetic counselor. They do not understand why all of their sons
are colorblind (an X-linked trait) but none of their daughters are colorblind. Both of the
parents appear normal. How would you explain this to the parents?
Suggested answer: Sons always get their X chromosome from their mother and the Y from
their father. In this case, the mother was the carrier of colorblindness. She appeared normal,
as did the father (who did not have colorblindness). When they had children, half of mom’s
gametes contained the X chromosome with the trait for colorblindness. When fertilized by a
Y chromosome, sons were produced. Half of them will be colorblind. For the daughters to
be colorblind, they would have had to receive an afflicted X from both their mother and
their father. (Red-green colorblindness is a recessive trait.)
2. Cystic fibrosis is a recessive genetic disease. Two parents do not have cystic
fibrosis, however their child does. Fully explain how this could have happened.
Suggested answer: In this case, both parents are carriers of the defective allele. Since this is
a recessive trait, they appear normal; however, half of their gametes contain the defective
allele. When an egg with the defective allele is fertilized by a sperm with a defective allele,
the embryo will have two defective (recessive) copies and will have the disease. The odds of
these heterozygous parents producing a child with cystic fibrosis are 25%.
3. A mother is unsure who the father is of her newborn. Before doing DNA analysis, a
simple blood test is performed to give some preliminary information. In this case, two
men are the potential father so their blood is also collected. Here is the summary:
mom—O, the baby—B, potential father 1—A, potential father 2—AB. Which man is
the most likely father? Write an explanation to justify your answer.
Suggested answer: The mom is type O, which is recessive (ii). Since the baby is type B
blood, his genotype must be (IBi). The real father had to have contributed the IB allele since
mom contributed the i allele. Potential father 2 is the only possible dad in this case who has
the IB allele to give.
Biology and Society
4. Why would it be difficult to develop a genetic test for a polygenic disorder?
Some issues and questions to consider: For polygenic traits, different mutations in different
genes may have the same or similar phenotypic effects. It would be difficult to test for all
possible mutations for all genes involved. However, if a specific mutation is known to run in
a family, members of that family can be tested for that specific mutation.
5. As the Human Genome Project progresses, we are learning more and more about
genetic differences that cause genetic disease. Some people advocate the use of this
knowledge to ultimately develop gene therapy. The goal of gene therapy would be to
find alleles that are “faulty” and to correct them with normal alleles. Ideally, to “fix”
every cell in an individual, this process would need to occur in embryos. Let’s suppose
a couple found out that later in their embryo’s life, their baby would develop an
incurable disease such as Alzheimer’s disease. Would you advocate the use of gene
therapy in the embryo to correct the problem before the child was ever born? What if
the embryo did not appear likely to develop a genetic disease, but the parents decided
that they would really prefer that their child have blue eyes as opposed to brown. Would
you advocate the use of gene therapy in this case? Should the use of this therapy be
limited? Who should decide those limits?
Some issues and questions to consider: How do we decide what is “serious” versus what is
not critical? If the technology existed, should there be any reason not to use it in any
situation? Who should set the limitations to prevent abuse of the technology?